We have been through a lot when it comes to tire sizing regulations and how to keep our staffs and customers informed. From numeric, to alpha-numeric, to all of the metric options, it hasn’t always been easy to explain what all of this means to the consumer, often leaving them somewhat confused.

All of these changes to tire sizing nomenclature – and size changes themselves – took place in the last 50 years, a relatively short period of time.

How about a quick review before diving into this subject? A P-metric sized tire always has a “P” at the beginning of its size description, such as P255/40R18 while the Euro metric system leaves out the “P”. Euro metric is the older method for measuring tires.

Over the last few years, we have noticed another tire sizing trend that is slowly finding its way in our country: Euro metric overtaking P-metric. Connected to the shift to performance tires on sporty cars, this trend won’t happen overnight, but dealers must start now to understand what this means to you, your employees and your customers.

If you want proof, look at any tiremaker’s databook from five years ago and compare it to this year’s guide. You will see a far greater percentage of tires offered today in Euro metric sizing.

First, don’t be alarmed. The dimensional difference between a P-metric P205/55R16 and a Euro metric 205/55R16 is, in theory, infinitesimal. But some important distinctions can apply. For instance, within a particular line of one tire brand, the OD is exactly the same for both sizes, but things like tire weight and original tread depth are different.

With that same brand and line, in P215/60R16 and 215/60R16 sizes, there is a difference in ODs and weight. Again, the differences are slight, but there are often enough differences between the two seemingly identical sizes to warrant you taking a few extra minutes to confirm that you have the right tire for the customer.

Thinking Inside the Box
There is another way to look at this. Picture a box. Inside the box picture an “X” in the upper left hand corner and another “X” in the lower right hand corner. One “X” represents one tiremaker, the other represents another tiremaker. What’s important here is that this box, or window, of specifications is where virtually every passenger tire built in the world must fall.

Those tiremakers with specifications that fall outside the window are ignoring dimensional standardization, which may cause a problem for you. And, for a fact, they do exist. For example, a tire produced outside the box could explain why a customer’s complaints may be valid when it comes to ride comfort.

But let’s be specific. This isn’t necessarily dangerous, but it is troublesome. In fact, the primary difference between Euro metric and P-metric tire sizes is the difference in the load calculation used for the two standards. That is to say, Euro metric size load capacities are defined by the European Tyre & Rim Technical Organization (ETRTO in Europe). P-metric load capacities are defined by the Tire & Rim Association in the U.S.

There are other tire standards organizations around the world, but the vast majority of tires sold in North America are based on one of these two standards.

Automobile manufacturers make the decision to use either P-metric or Euro metric tires for their new vehicles, and to a large degree they have forced consolidation. The good news is that all new tire sizes introduced since 2006 have harmonized loads, so there is no difference in load capacity in these sizes. Many of these new sizes fall in the UHP tire category.

In addition to some minor dimensional differences, the same “size” Euro metric and P-metric tires may also have small differences in the inflation pressure and load capacity. Therefore, if two tires have the same measurement and the same speed rating, the two are considered the same and can be used on the same axle or in sets of four. Just follow the carmaker’s inflation recommendation.

While Load Index has become a common part of sizing nomenclature (for example, P255/40R18 91V), NHTSA does not recognize the load index numbers as such. The actual load capacities recognized by NHTSA are based on the particular standard that was used for that tire. That load is the maximum load value stamped on the tire sidewall in units of pounds and kilograms.

Whether it’s P-metric or Euro metric, when selecting replacement tires for a vehicle, those tires must have sufficient load carrying capacity for that vehicle at the manufacturer’s inflation pressure.

From here it gets a bit hairy. Example: Many Euro metric tires have slightly higher load carrying capacities stamped on the sidewall than similar sized P-metric tires. In practice, though, many P-metric tires actually have higher load capacities, particularly when operated at lower than recommended inflation pressures, such as 28 psi or 30 psi.

This is a primary reason that some Euro metric sizes are not always an automatic replacement for a similar sized P-metric tire.
 
Confused? It’s OK. Let’s try to simplify things a bit. The nice thing about checking ETRTO listings for standard load tires is the load capacities aren’t specific to a tire size. You don’t read a load and inflation table by looking up a size, instead you read down to a load-index value, read across to an inflation pressure and there’s your load capacity.

But you’d better be certain you’re in the right table, because the tables for standard load and extra load (XL) are completely different.

Summing it All Up
So just how close are P-metrics and Euro metrics? “For nominal sizes, with only slight differences, we could probably make the same Euro metric and P-metric sizes in the same mold,” said Michelin.

“Our major concern is mixing tires. Do not put a P-metric size tire and a Euro metric size tire on the same axle, and preferably use one size or the other on the same vehicle. In other words, the customer will be happier if all four tires come from the same sizing system.

“We have observed minute changes in dimensions between Euro metric and P-metric and we have seen more significant changes in light truck tires. The best way to avoid problems is to look at your manufacturer’s dimensional and inflation tables,” Michelin said.

As noted earlier, there are but minor dimensional differences between P-metric and Euro metric sizes. There is, however, considerable difference between a Euro metric and an LT-metric light truck size. Here again, you must refer carefully to the manufacturer’s load and inflation tables, and make certain you are reading the right table for the right tire and the right application.

Is there a trend for growth in Euro metric sizing for light-truck tires? Yes, but more slowly than the shift away from P-metrics. Michelin also advises dealers to be much more concerned about the mixing of sizing systems on AWD vehicles. This difficulty is related not to load but concern for tranny rpm mix-up. We’ll look at this in a future issue.

Finally, be sure to use tire and vehicle manufacturer’s data, paying particular attention to dimensional data, load-carrying capacity, and inflation pressure capable of carrying the load.

In the meantime, I’m sure you can imagine how moving completely to one size designation could reduce the number of SKUs you have to deal with. Perhaps this slow slide to Euro metric sizing can help relieve that stress.